AJ,
"Subsistit in" is not heretical when properly understood according to the Church's Magisterium.
In 2007 the CDF declared that, "“The use of this expression, which indicates the full identity of the Church of Christ with the Catholic Church, does not change the doctrine on the Church.”
Source: Congregation for the Doctrine of the Faith, Responses to Some Questions Regarding Certain Aspects of The Doctrine on the Church (2007), Response to Third Question.
The CDF makes clear that this phrase and others were subject to misunderstanding and, with the 2007 explanations, sought to clear the air.
"Given the universality of Catholic doctrine on the Church, the Congregation wishes to respond to these questions by clarifying the authentic meaning of some ecclesiological expressions used by the magisterium which are open to misunderstanding in the theological debate."
Thus the term "subsistit in" is congruent with previous Catholic teaching.
I hope this helps.
God bless.